Orthodontic appliances are known for repositioning teeth so as to achieve optimum formation of the maxillary and mandibular dental arches. Sometimes expansion of the dental arches is required and other times contraction thereof is required. Orthodontic archwires are used to move teeth, but are not generally suitable to expand or constrict a dental arch. This may be accomplished using transpalatal arches or expansion or constriction appliances.
One such device, the transpalatal arch bar, can be used to expand or contract the dental arches and to rotate, intrude, extrude and/or torque particular teeth in a patient. Transpalatal arch bars and similar related appliances are shown in U.S. Pat. Nos. 3,792,529, 4,592,725, 4,815,968 and 4,886,451. These devices are typically formed of relatively stiff (high modulus of elasticity) stainless steel wire having a U-shaped loop segment or a torsion-spring coil convolution, to supply the desired corrective forces. One drawback associated with such devices is that the spring-like forces supplied by the device are initially high but decrease a the expansion takes place and thus the device must be re-adjusted or replaced at periodic intervals until the desired tooth movement has been achieved.
Another type of device known for correcting transverse dental arch discrepancy is the rapid palatal expander (RPE). Such devices may be used to expand the maxillary arch. The device is affixed, i.e., either bonded or banded, to opposing teeth in the maxillary arch, typically first molars and/or first bicuspids. The transpalatal portion of the device has an expansion screw which the patient turns a predetermined amount each day. Turning the expansion screw in the palatal expander supplies the expansive forces of the device. As with the transpalatal arch bar, the initial force is high, but rapidly decreases as the teeth are moved.
Rapid palatal expanders have several drawbacks, including one or more of the following. The device provides a single force which tends to tip teeth. The forces exerted may be initially as high as 11 lbs., which causes tissue destruction and patient discomfort. Since the devices are patient activated, the cooperation of the patient is required, as is proper training. Furthermore, these devices tend to be bulky and therefore obtrusive to the tongue, which may affect speech and which may adversely affect oral hygiene. Additionally, these devices cannot be fabricated at chair side by the clinician.